There is a lot of work out there for a plumber but alass
You will not be a fully qualified plumber unless you have done an apprenticeship, 4 years training and college work. Then if you want to go onto Gas its even more college and work experiance aswell as umpteen examinations and practical tests.
These courses only introduce you to plumbing and teach the very basics of it. You will have no way learnt anything at all in reality. And you would never be taken on as a plumber without all the qualifications and training + practical experiance.
And seems as though to work on plumbing and gas you have to by law have these qualifications and experiance. I fail to see why anbody would go on one of them. except for people who would like to learn a bit of DIY.